Deeper into the Forbidden City

In last night’s 5e Dwellers of the Forbidden City game, the players were reunited with their lost Warlock (who had been presumed killed when the Temple of Greed collapsed upon her and many of the rest of the party the day before) and then headed towards the swamp lair of the “civilized” frogmen (Bullywugs).

Last game they had managed to get into a fight with a collection of barbarian bullywugs, so they were feeling a little beset by hostile forces. Bullywugs in the swamps, Tasloi in the buildings around them, and Yuan-Ti patrols in the streets.

Heading north through the city they distracted a tasloi guard post by lighting their gardens on fire, charmed a tasloi fire brigade member, and then headed into the unclaimed no man’s land on the edge of the swamp – a part of the city almost entirely collapsed and in ruins. In the ruins they were ambushed by a bugbear scavenging team and finally made their way to the Bullywug stockade a little the worse for wear (and with the warlock out of spells).

The bullywugs greeted them, sat them on sedan chairs and jumped them over the stockade walls where they were introduced to King Groak who explained (finally) the political situation of the Forbidden City and the various factions living within it – the Mongrelmen to the northwest, the Bullywugs and their God in the swamps, the Tasloi and Bugbear servants of the Yuan Ti and of course the small number of actual Yuan Ti who have had to change their raiding and scavenging operations to the south because massive monsters have been disgorged from the Iron Temple to the north, infringing on their traditional territories.

When they explained that they were spies for a powerful confederation of human cities, and were also looking for a lost chief’s son, King Groak offered them a deal – go to the burning halls where the Bullywugs can’t travel and bring back the token of their god (a pearl about a foot in diameter) and they will help with the rescue of the young lad. To sweeten the deal they explained that the group who currently have the pearl also have a spear that has incredible magics for slaying the Yuan Ti that the party will also probably find useful.

The game ended in a lava tube north of the Forbidden City, where the party had just dispatched a hellhound and forced its packmate to run away deeper into the lava chambers…

I have a question about those geomorph coasters:
How are you using those at the game table?
Are they hidden behind your screen? Are they in the open on the table?
Are they part of the module or are they just – coasters?